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Amargasaurus was small for a sauropod, measuring 9[3]:4[4]:172 to 10[5]:304 meters in length and weighing approximately 2.6 tons.[3]:4 It moved on four legs and probably was unable to rear on its hind legs.[6]:344 The neck of Amargasaurus, measuring 2.4 meters in length, was proportionally short for sauropod standards.[7]:5, 6 It consisted of 13 cervical vertebrae which were opisthocoelous (convex at the front and hollow at the back), forming ball-and-socket joints with neighbouring vertebrae.[4]:174 The trunk was made out of 9 dorsal and probably 5 fused sacral vertebrae.[4]:174 While the foremost dorsals were opisthocoelous, the remaining dorsals were amphyplatyan (flat on both ends).[4]:174 Robust transverse processes (lateral projections connecting to the ribs) indicate a strong developed rib cage.[6]:339 The dorsal vertebrae of Amargasaurus and other dicraeosaurids lack pleurocoels, the deep lateral excavations that were characteristic for other sauropods.[6]:339

The most obvious feature of the Amargasaurus' skeleton were the extremely tall, upwardly projecting neural spines on the neck and foremost back vertebrae.[4]:174 The neural spines were bifurcated along their entire length, forming a double row.[4]:174 They were circular in cross section and tapered towards their tips.[4]:174 The tallest spines could be found on the middle part of the neck, where they reached 60 cm on the 8th cervical.[4]:174 On the neck, they were bowed backwards, projecting above the adjacent vertebra.[5]:304 Greatly elongated spines continue along the last two dorsal vertebrae, the hip and foremost tail. However, in these regions the spines were not bifurcated but flared into a paddle-shaped upper end.[8]:1139

The skull is only incompletely preserved––however, it likely had a horselike, broad snout equipped with pencil-like teeth, based on related sauropods for which more complete skulls are known.[9] As in other dicraeosaurids, the external nares (nostril openings) were situated in the posterior half of the skull, diagonally above the eye sockets, which were proportionally large.[6]:338[10]:341 Additional (fenestrae) could be found surrounding the eye sockets: The infratemporal fenestrae, located below the eye sockets, were long and narrow.[10]:341 Behind the eye sockets were the supratemporal feneatrae, which, in dicraeosaurids, were uniquely small and directed laterally (in contrast to other diapsids, where they were directed upwards).[4]:172 In front of the eyes would have been the antorbital fenestra, although this region is not preserved. Small openings, called parietal openings or fontanelles, can be seen on the backside of the skull. In other tetrapods, these openings are usually seen only in juveniles and they close as the individual grows.[11]:212

This site is located in the lower (older) sections of the La Amarga Formation, which dates to the Barremian through early Aptian stages of the Early Cretaceous Period, or around 130 to 120 million years ago.

Amargasaurus is known from a relatively complete skeleton from a single individual. This skeleton includes the back of the skull, and all vertebrae of the neck, back, and hips, as well as a bit of the tail. The right side of the shoulder girdle is also known, as are the left forelimb and hind limb, and the left ilium, a bone of the pelvis.

Gregory Paul argued that parallel neck sails would have reduced neck flexion. Instead, he proposed that, with their circular rather than flat cross-sections, these spines were more likely covered with a horny sheath. He even suggests that they could have been clattered together for a sound display.[1]

Similar spines are found on the presacral vertebrae of Dicraeosaurus from Africa, although not nearly as tall.